The roll-out of the previous update - GDR2 - isn't even complete (thank you, carriers), and Microsoft is already pushing out the next one, GDR3. This update for Windows Phone 8 is the last one before 8.1 comes out next year, and brings with it a number of small improvements, such a close button in the multitasking view, a driving mode, support for newer hardware and 1080p displays, a rotation lock, and more.

There's no telling as of yet when Windows Phone 8 users will be getting the update, but non-branded phones will most likely get it first. On top of that, if you're a Windows Phone developer, you can get the update straight away.

What carriers mainly want are either differentiation or easy customers/sales.

Apple has one of the most effective marketing engines in the world. That means iPhones basically sell themselves, as far as the carriers are concerned. Which is why Apple can negotiate from a strong position.

Other brands/ecosystems require a bit more effort from the carrier to push to the end customers, which means the manufacturers of those products have to buck up to the carriers demands for differentiation via branding/bloatware/experience/etc.